Better continuity of service when moving across a networkTerminal-based interference solutions benefit operators by providing:

Higher total capacity

Improved coverageThese benefits will be described in detail in section 3.2. Receiver design2.1. Interference mitigation in LTEThere are numerous publications on the topic of interferencecancelation techniques. For instance, [IRC-GSM] deals with interferencemitigation for 2G systems, and the 3GPP has defined and standardizedUE receiver classes related to interference cancelation capability for 3Gsystems (WCDMA) [TR25.963] for several receiver types as defined intable 2.Similarly, the 3GPP has recently begun to investigate interference-aware receivers for Release 11, as described in [TR36.829].While the theoretical aspects and algorithm principles of interferencecancelation are well understood, numerous challenges lie in theimplementation of these techniques in an LTE terminal:

First, the LTE waveform is based on OFDMA modulation, which isnot the same as 2G/3G modulation. New techniques have had tobe developed because channel estimation and receiver design forthe multi-carrier OFDMA modulation of LTE is very different fromthat used for the single-carrier and WCDMA modulation of 2G/3G.Practical implementation of interference mitigation theory in LTErequires intimate knowledge of OFDMA architecture.

Second, LTE throughput is significantly higher than 2G/3G throughput,but the overall budget for power consumption is constrained in orderto meet the requirements of battery-powered devices. Therefore,the implementation of interference mitigation techniques must bedesigned to require minimal hardware resources and consume minimalpower. This goal can be achieved only by accounting for interferencemitigation in the modem architecture from the initial design.Finally, the LTE standard has defined several transmission modes (seeTable 3), from which specific interference mitigation techniques mustbe derived. Specific terminal feedback information transmitted to theeNodeB (for dynamic throughput optimization) must be taken intoaccount in the design of interference mitigation techniques.3GPP NameReference receiverType 0=RAKEType 1=Diversity receiver (RAKE)Type 2=EqualizerType 2i=Equalizer with interference awarenessType 3=Diversity equalizerType 3i=Diversity equalizer with

Nonlinear:In this approach, the interfering signal is estimated andthen subtracted from the received signal, possibly in an iterativemanner. This requires explicit modeling of the interfering signal. Suchan approach provides excellent performance, but is very sensitive toerrors in the estimation of the interfering signal.

Linear:In this approach, the receiver uses multiple antennas toperform spatial suppression of the interfering signal. Specifically,the receiver forms a receive antenna beam, with a spatial null in thedirection of the interferer. This works best with a large number ofreceive antennas, but provided with proper spatial properties, thistechnique can handle a number of interferers. Such receivers areusually called IRC (interference rejection combiner) receivers.Note that for both of these approaches, channel estimation, wherebythe channel and interference are accurately estimated, is a key step inthe process. Another key step is to detect the presence or absence ofinterference. This eases overall processing in the UE, considering thatinterference is highly unpredictable and dependent on variable factorssuch as channel conditions, traffic from other terminals, and schedulingfrom the eNodeB.2.3. Introducing Sequans AIRBased on the specific requirements of interference mitigation inLTE, and considering the rapidly changing interference conditionsin packet-switched networks, Sequans has designed Sequans AIR(active interference rejection), a compact LTE receiver that includesinterference mitigation capability, suited to the various transmissionmodes of LTE. Sequans AIR adopts the linear approach to interferencemitigation. It has been co-developed with technology partnerArrayComm, a pioneer in antenna processing and interferencemanagement techniques. Sequans has leveraged its own expertise withOFDMA and MIMO receivers, and the combined efforts of Sequans andArrayComm have resulted in an innovative and powerful interferencemitigation algorithm and an optimized implementation on silicon.Sequans AIR has been designed to mitigate the interference not onlyfrom data channels but also from control channels. Even though controlchannels are designed to be more robust than data channels, they mayalso suffer from strong interference. If they do, the terminal may not beable to demodulate the control channel and may lose its connection tothe network.2.4. Support on Sequans productsSequans AIR is designed for use on Sequans’ latest LTE platforms:

The link performance data provides good information about thereceiver performance in a range of interference conditions (fromnoise-limited to interference-limited). These results do not directlytranslate to show the benefits of Sequans AIR in a real system.

The system level results were obtained through partnership withSIRADEL, a leading provider of advanced RF tools, using realisticgeographical data. These results clearly demonstrate the benefits ofSequans AIR in real operational deployment conditions.3.1. Link level performanceIn order to evaluate the benefit of Sequans AIR in the receiver, theSequans AIR algorithm was implemented in Sequans’ LTE simulator,which is bit accurate and can represent the true performance ofthe chip. Only downlink is considered in these simulations. The firsttwo simulations assume an ideal link adaptation where the bestMCS (modulation and coding scheme) per SNR (signal to noise ratio)point is selected independently for a Sequans AIR receiver and areference implementation for an industry-standard MRC (maximumratio combining) receiver. The channel considered is the extended-vehicular-A channel in low mobility as defined by the 3GPP. Thecarrier frequency is 2.6 GHz and we assume cell planning such thatthe interfering cell(s) and the serving cell reference signals are non-overlapping. We assume up to three interfering cells in the link layersimulation.With respect to the interference profile, we assume that theinterference consists of data transmitted in either the same transmitmode as the useful data from the serving eNodeB, or using a differenttransmit mode. In all cases, the downlink sub-frames are fully allocated,from both the serving and interfering sides. In the case of the PDCCH(physical downlink control channel), we assume for the sake ofsimulation that the serving and interfering eNodeBs consider the sameaggregation level.The next figures illustrate the performance of the AIR receivercompared to a reference MRC receiver. In Figure 5, we consider a singleinterferer, having a constant C/I (carrier to interferer) ratio of –3dB.This means that the power level of the interferer is twice the powerlevel of the useful signal. The useful signal is using TM2 (the most robustway to transmit information within the various transmission modes),while the interferer is using TM1.In this scenario, the MRC receiver has a throughput floor of about 10Mb/s while the AIR receiver yields much higher throughput up to 35Mb/s or about 350 percent of the reference MRC. In this scenario, evenwith a good SNR, the performance is interference-limited for the MRCand the AIR receiver therefore provides much higher throughput.Figure 6 presents a very challenging scenario with three interferers.The first interferer has the same power as the serving cell while thesecond has power 3dB below it and the third, 6dB below. Even in thischallenging scenario that requires rejecting interference from threeinterferers with only two UE antennas, the AIR receiver provides about35 percent higher throughput than the reference MRC receiver.Figure 5 - Single-interferer link-level PDSCH performance−10−505101520250510152025303540SNR (dB)Throughput (Mbps)

The network is interference-limited. Even at good SNR levels, thethroughput drops considerably with the default receiver as comparedto the Sequans AIR receiver.

A standard receiver may not be able to connect even in a deploymentthat was designed for capacity (i.e. over-dimensioned with respect tocoverage).

A receiver able to mitigate interference can recover most of thedegradation in a realistic deployment.A similar result may be seen for indoor users as illustrated in Figure 13,although the number of areas with no service is even larger (reflectingthe cumulative effects of interference and the loss of signal strength).However, in the deep indoor areas where the SNR is much affected bythe indoor penetration losses, the performance of the default receiverand the Sequans AIR receiver become close. In this case communicationis no longer interference-limited, but noise-limited.Finally, in a scenario with a lower level of interference (neighboringcells have a traffic load of 50 percent), the relative gain of the SequansAIR receiver compared to the default MRC receiver is lower than inthe full interference scenario, as illustrated in Figure 14. Nonetheless,the Sequans AIR receiver is able to recover the interference-freeperformance, except in the few areas with a very low C/I.4. ConclusionInterference is a key issue in LTE networks. Solutions implementedat the terminal side provide key benefits, both for end users and LTEoperators. Sequans AIR is a solution that has been designed to fit onSequans’ chipset architecture, with proper hardware accelerators toenable full line-rate performance. Sequans AIR works in both TDDand FDD modes for all of the transmission modes defined in the LTEstandard. The benefits of Sequans AIR have been proven, both at link-level and system-level.5. AcknowledgementsSequans wishes to thank its key technology partners, ArrayComm andSiradel.5.1. ArrayCommArrayComm is a provider of physical layer solutions for wirelessinfrastructure and client device applications. ArrayComm is a worldleader in multi-antenna signal processing, delivering commercialA-MAS™ software now that combines MIMO, beamforming, andinterference cancelation to improve end user experience and radionetwork economics through gains in coverage, client data rates, andsystem capacity. The company’s comprehensive and flexible PHYsolutions include optimized DSP software and hardware acceleratorsthat save development costs and time-to-market.

www.arraycomm.com5.2. SiradelSIRADEL (www.siradel.com) is a high-tech company (small-mediumenterprise) created in 1994 and based in France, China (Hong-Kong) andCanada (Toronto). Siradel provides products and services for the ICTIndustry in particular wireless telecommunications.The portfolio of the company is composed of: